There are circuit arrangements in which current sources are used in order to register accurately an input signal applied to the circuit arrangement. In this context, a current source generates two electrical currents with opposite bias, so that, within the circuit arrangement, a first current source and a second current source, designated in the following as the current sink, with opposite bias to the first current source, are introduced into the circuit arrangement.
Such a circuit arrangement is, for example, a so-called common-gate amplifier in which the control terminal of an input transistor is regulated to a reference potential. In order to ensure that such circuit arrangements are used correctly at their operating point, it is necessary to introduce into the circuit arrangement an ideal current source and an ideal current sink with opposite bias to the current source, so that the current generated in the current source is completely absorbed in the current sink, so that a total current from the current source and the current sink ideally provides a magnitude of exactly zero amperes. In this manner, an input signal to be registered by the circuit arrangement, for example, an input current, is not registered with a falsification resulting from an unmatched error signal, for example, an error current.
If a current generated by the current source is not completely absorbed by the current sink or if the current absorbable by the current sink is greater than the current generated by the current source, a total current is provided, which is not equal to zero amperes, designated in the following as an error current. This error current is added to the input signal. Such mismatch between the current source and the current sink is specially troublesome when generated by time-dependent or temperature-dependent fluctuations of the current sources and have a direct influence on the input signal to be registered.
If a circuit arrangement of this kind is used as an input unit in a measuring instrument operating with high precision, such error currents influence the measurement result of the measuring instrument in such a manner that measurement errors occur. With measuring instruments such as digital storage oscilloscopes or spectrum analysers, the registration of the input signal must be implemented without error over several decades of the measurement amplitude to be registered. Furthermore, the registration should be possible over a large frequency range of the input signal, in particular, it should be possible to register through a circuit arrangement input currents from DC current through to alternating currents of high-frequency.
With the common-gate amplifiers known hitherto, the current flowing through the input transistor must therefore be substantially larger than the maximum input current to be registered. However, applications are conceivable in which an input current to be registered is much smaller than the current through the transistor. Such input currents can be registered only inaccurately with circuit arrangements of this kind.
Regardless of the occurrence of error currents resulting from a current source or respectively current sink embodied in a non-ideal manner, it is necessary to deliver a circuit arrangement which provides a low noise behaviour. Because of the use of two different current sources in the circuit arrangement, the addition of additional noise cannot be prevented. Through the use of a first current source separate from a second current source operated as a current sink, two different noise sources are introduced into the circuit arrangement, wherein the two noise sources are not correlated with one another. Accordingly, these two noise sources are statistically independent of one another and impair the signal-noise ratio of the circuit arrangement independently of one another.
Current mirrors for the generation of current sources are known, for example, from EP 2 533 128 B1, wherein the current sources can also always be used as current sinks. In this context, a reference current is used, which is duplicated by two output current mirrors. These output current mirrors are built up, for example with n-channel-field effect transistors. The current generated from one of the output current mirrors is supplied to a further current mirror made up from p-channel field effect transistors.
The generated currents of the p-channel or respectively n-channel current mirror are then identical to one another if all of the transistors used have the same dimensions and are perfectly matched with one another. With the use of current mirrors as current sources or respectively current sinks, it should be noted that the source-drain conductance is very low in order to avoid errors caused by different drain-source voltages (abbreviation VDS). Such current sources can also be constructed from cascoded transistors or control elements in order to adapt the drain-source voltages of critically matched transistor pairs.
These current sources have an accuracy up to one percent, that is to say, the currents are generated with a maximal accuracy of one percent. Such an accuracy is too low for the input units of measuring instruments. In order to achieve an improvement in accuracy, calibration circuits must be provided to match the current sink to the current source. With such a calibration circuit, it must also be taken into account that the transistors to be calibrated are strongly temperature dependent, so that it is almost impossible to find an appropriate calibration which applies across a wide temperature range.
Furthermore, with current sources and current sinks constructed by means of current mirror circuits, it is almost impossible to obtain a low noise factor. Since different current sources in a circuit arrangement are delivered by different current mirrors, each of the current sources forms a statistically independent noise source which further impairs the signal-noise ratio of an input signal.
One object of the present invention among others is therefore to deliver a high-precision current source and a circuit arrangement with this current source, which can generate a large number of currents and is low in noise.